1. Field of the Invention
This invention relates to video signal processing circuits. More particularly, it relates to such circuits in which video signals, for example, from a video tape recorder, are subjected to error correction, error concealment and weighted mean processing.
2. Description of the Prior Art
In recording and/or reproducing digital sample data of video signals by means of a digital video tape recorder (VTR), so-called "code errors" result from noise or defects in the recording medium. The erroneous sample data produced by the code errors are subjected to error correction using error correction codes and those which cannot be thus corrected are subjected to error concealment by interpolation or replacement using other error-free sample data. In generating a sequence of odd and even numbered fields which have been disrupted as a consequence of reproduction at a speed different from that employed during recording, weighted mean processing is frequently performed in which signals of a given field are used to form signals of another field.
In the so-called "D-1" format (a 4:2:2 format set forth in CCIR recommendations No. 601), which is one of the formats used in component digital VTR, two-dimensional error correction using a product code is performed, which involves the use so-called "outer"and "inner" codes. During recording, 360-byte-per-line sample data are subjected to in-line shuffling, and then are fed to a two-dimensional error correction coding circuit where product codes are added thereto. First, a 2-bit outer code, i.e. outer correction code or outer purity, is annexed at intervals of 30 bytes or samples. The resulting code block is subjected to sector array shuffling and then a 4-byte inner code, i.e., inner correction code or inner party, is annexed at intervals of 60-bytes-per-row of sample data in the perpendicular direction of the shuffled code block to form an inner code block. A sync block structure generated from two such inner code blocks is recorded as a unit on the magnetic tape. During playback, an operation which is the reverse of the above mentioned operation is performed. Thus, two of the inner code blocks are removed from the sync block and subjected to error correction with the use of the inner code, followed by deshuffling which is the reverse of the above described sector array shuffling. Finally, a so-called erasure correction is performed, using 2-byte outer codes at intervals of each outer code block having 32 bytes per row.
Any erroneous sample data, which could not be error corrected through the use of the product code, are subjected to error concealment by interpolation or replacement. Among the methods for error concealment are (1) a method of interpolation in the horizontal or H direction, using sample data on both sides of and on the same line as the erroneous sample data, (2) a method of interpolation in the vertical or "V" direction, using sample data at the same horizontal position as and on lines above and below the line of the erroneous sample data, (3) a method of interpolation in the D.sub.+ direction, using near-by sample data on a positive diagonal line of the reproduced video image, that is, a diagonal line with a rightward positive gradient, (4) a method of interpolation in the D.sub.- direction, using near-by sample data on a negative diagonal, that is, on a diagonal with a negative gradient from left to right, (5) a method of replacement with error-free samples of the preceding frame or field exhibiting high temporal correlation, and (6) a method of replacing the erroneous sample data with near-by sample data.
Meanwhile, it is desirable for actual error concealment to function over a wide range of error rates, for example, over a range from substantially 0% during normal reproduction to substantially 100% during high speed tape shuttling.
However, an error concealment method with a high concealment accuracy may operate properly for a low error rate but fail to operate properly for a high error rate, while an error concealment method which is usable even at a high error rate may suffer from low concealment accuracy, such that no single concealment method is fully effective o conceal errors with suitable or acceptable concealment accuracy over the above mentioned wide range of error rates. Even if it were possible to use a plurality of error concealment methods in combination and to provide the ability to switch manually from one to the other manual switching is cumbersome and undesirable from the standpoint of operational reliability.
On the other hand, in case of a high error rate in the sample data reproduced by the VTR, most of the sample data are erroneous and thus cannot be error concealed by interpolation or replacement. It is however desirable that some sort of error concealment be carried out even on these occasions.
Additionally, it occurs frequently that correct sample data are contained in sample data which as a whole have been determined to be in error in the course of the error correction process. For example, in the course of the above-mentioned outer error correction, when the number of erasure pointers generated by the inner error correction product code exceeds the number of parities of the outer code, it is possible to compute the syndrome of the outer code block without performing error correction thereon, when all of these syndromes are determined to be "0", under the assumption that all of the sample data in the outer code block are free of errors. However, in employing the inner code or inner parity, it is possible that erroneous data will be deemed to be correct since it is not certain that the inner code or parity has sufficient error detection capability to generate correct erasure pointers, or if error detection by the outer code or parity exhibits a sufficient reliability. It is undesirable from the viewpoint of reliability to treat such sample data as being error free, while it is wasteful to discard it as erroneous.
When weighted mean processing is to be performed in order to synthesize signals of one field from signals of a given or existing field after error concealment, several horizontal delay devices are required for error concealment, and especially for two-dimensional error concealment, while several other horizontal delay elements are required for weighted mean processing. However, the use of dedicated horizontal delay devices for error concealment and weighted mean processing would be uneconomical.
Finally, when weighted mean processing operations are performed at the upper and lower ends of the picture surface the image quality may deteriorate and the picture may suffer from flicker or vertical movement.